The present invention relates to a jackscrew for use in correcting misalignments of teeth, comprising two main bodies. A jackscrew of this kind is known from German Patent 824 832. The known jackscrew serves, in conjunction with a split palate plate, to effect a correction into the correct tooth position in the event of an incorrect tooth position, for example if teeth in the upper or lower jaw project into the oral cavity or are displaced laterally inward. The two parts of the palate plate are spanned by the jackscrew and forced apart by adjusting the jackscrew. For this purpose, the known jackscrew has two bodies whose spacing relative to one other can be varied by means of a double spindle. Each of these bodies is usually embedded in one of the palate plates. The pressure which is built up by actuation of the spindles is cushioned by compression springs which act between the spindles and the two bodies that are to be forced apart. In the case of the known jackscrew, the springs are helical springs which surround the two threaded sections of the spindle and are braced at their one end against a cover which closes off a recess of the respective body so as to brace at their other end against a buttress in the form of a threaded sleeve which is threaded onto the relevant threaded section of the spindle and, secured against rotation, is displaced in the relevant recess of the body upon actuation of the spindle. A disadvantage in this context is the fact that in accordance with Hooke""s law, the spring force weakens as the treatment progresses. As a result, the effectiveness of the jackscrew also weakens during the course of the treatment, and the tooth position correction slows down and comes entirely to a standstill whenever the jackscrew is not readjusted. To avoid the need for frequent readjustment of the jackscrew, it is easy to give in to the temptation to load the springs in the jackscrew more strongly than is good for the teeth and the tooth position.
With the known jackscrew, it was moreover possible for deposits of food residues and tartar to penetrate into the jackscrew and cause jamming and immobilization of the screw.
It is the object of the present invention to indicate a way to make such jackscrews easier to work with, it being desirable on the one hand not to need to readjust the jackscrew so often, but on the other hand not to create the risk of shear stress peaks on the teeth.
This object is achieved by means of a jackscrew comprising two main bodies whose mutual separation is adjustable by means of a spindle provided with an operating part, and continuous with that part, one or two screw-thread sections, the operating part being rotatably mounted in one body, and one screw-thread section being rotatably mounted in the other body, further comprising straight-line guide elements which engage with both bodies and guide them in a straight line while also preventing any relative rotation when the separation between the bodies is altered, and a thread sleeve on each of the screw-thread sections of the spindle, the threaded sleeve being guided inside a recess in the respective body and along the spindle so as not to rotate relative to the body and transmitting its sliding motion to the bodies via a spring which determines the expansion force, characterized in that the spring is made from a shape-memory alloy which is pseudo-elastic at temperatures prevailing inside the mouth. The new jackscrew uses as the spring not a conventional steel spring but a spring made of a shape memory alloy that is pseudoelastic at the temperatures prevailing in the mouth. Preferred shape memory alloys are alloys based on nickel and titanium, in which nickel and titanium are contained in approximately equal atomic percentages. Alloys of this kind can exist, depending on the temperature selected, in either the austenitic or martensitic state. Martensite is present at lower temperatures, austenite at higher temperatures. The temperature at which the alloy begins to convert, upon cooling, from austenite into martensite is also referred to as the Ms point. In the martensitic state below the Ms point, alloys of this kind can exhibit shape memory: a plastic deformation occurring in the martensitic state can be reversed above the Ms point. A shape memory alloy of this kind can exhibit pseudoelastic behavior in a temperature range just above the Ms point. Pseudoelastic behavior is characterized in that the force required for an increasing elongation initially rises sharply as expected for an austenite, but then, after reaching an elongation of approximately 1 to 2%, increases only slightly as elongation progresses, and rises steeply again only after greater elongations (of 6 to 8%) are attained. The middle elongation region is referred to as the xe2x80x9cmartensite plateau.xe2x80x9d The name derives from the fact that martensite forms in the alloy in response to the tensile stress. When tension on the material is relaxed, it returns to the austenitic state. These pseudoelastic elongations are reversible to a large extent up to elongations greater than 6 to 8%. Because of the pronounced martensite plateau, pseudoelasticity does not obey Hooke""s law. As a result, springs which behave in this pseudoelastic fashion are particularly suitable for the purposes of the present invention, since for spring travels in the martensite plateau range, the return force of the spring is almost independent of the spring travel. A jackscrew according to the invention thus has the great advantage that tensile stress remains almost unchanged over the course of the treatment. Because of the consistent spring load, the teeth are aligned more quickly than hitherto, and readjustment of the jackscrew is not required as often as in the existing art. Since there is almost no change in the spring force as long as it remains within the martensite plateau, it is moreover possible, by applying the invention, to reliably prevent an orthodontist from inadvertently applying excessive loads during a treatment, since simply by matching the spring travel to the elongation load of the jackscrew, it is possible to prevent any displacement beyond the martensite plateau from occurring; as a result, in the case of the jackscrew according to the invention, the tensile force is approximately determined only by the selection of the pseudoelastic spring, but not by the displacement travel of the spindle. The displacement travel of the spindle determines only the end of the tooth correction, not the force applied for it.
The invention makes the jackscrew substantially more convenient and reliable to use.
Pseudoelastic springs are known per se, but have hitherto not been used in jackscrews for orthodontic purposes, for which, according to the invention, they open up a substantially expanded field of application, putting an end once and for all to the hazards (damaging load peaks) associated with frequent readjustment of the jackscrew as required in the existing art.
Depending on the selected arrangement of the springs in the bodies of the jackscrew, the springs can be utilized as tension springs or compression springs. In terms of the construction of the jackscrew, an embodiment as compression springs is more favorable and therefore preferred.
The jackscrew could have a spindle with a head and a threaded portion, the head being mounted in a body and the threaded portion extending into the other body. Preferably, however, the spindle has two threaded portions which extend in opposite directions from an actuating member, located in the center, of the spindle, and have threads running in opposite directions. A configuration of this kind allows greater displacement travels.
In order to be effective, the spring must engage on the one hand on the body and on the other hand on the threaded sleeve located on the spindle. For this purpose, the recess in the body in which the threaded portion of the spindle, the threaded sleeve, and the spring are located is closed off by a cover whose installation completes assembly of the jackscrew. Said cover can be welded or soldered to the respective body, but it can also be pressed or set in and fastened by crimping over a rim provided on the body. Said cover then serves not only as a buttress for the spring, but also closes off the recess provided in the body at one end, so that food residues cannot penetrate from there, solidify, and thereby impair the function of the jackscrew. In order to prevent any penetration of food residues from the other side as well, a seal is preferably provided there, in particular an O-ring, which is arranged on the actuation member of the spindle in the vicinity thereof, and seals the access leading into the respective recess of the body from that end. If deposits of this kind, due to food residues, tartar, and accumulated plaque, are kept out of the recess in which the threaded sleeve and the pseudoelastic sleeve are located, proper functioning of the jackscrew throughout the course of the treatment can be ensured with no need for occasional cleaning operations to be performed.
The linear guide means are advantageously pins that are fixed in one of the two bodies and extend into bores of the other body. There is a certain risk of incrustation in the case of the linear guide means as well, although it is less than in the region of the spindle because the pins must slide out of the bores in which they are guided as the jackscrew is extended, but need not move inward; the pins also move back into the bores only over the length of the spring travel. It may nevertheless be advantageous to provide sealing rings there as well, which surround the pins and prevent the penetration of initially liquid food residues into the bores of the one body.
An exemplifying embodiment of the invention is shown in the attached drawings.